There are many "someone better"s, but hinata is kageyama's someone better. It was meant to be a specific person from the start.

In ch387, kageyama tried competing with and keeping up with oikawa and iwaizumi, but neither of them were meant to be his partner, they were each other's partners ("you're the partner I can be proud of").

Kageyama might have gone through the same thing in high school if hinata wasn't there. He would eventually have found players as good as him and better, but never a partner, even in karasuno.

The panel of him leaving shiratorizawa really gets to me, but it was for the better; he would never have synced with ushiwaka and the team like he did with hinata and karasuno.

Hinata needed kageyama to start reaching his potential and realising it. Kageyama needed hinata on an emotional level so much, he had been missing hinata his whole life until then. "What have you been doing for the last three years" is meant to make hinata realise that he is capable of incredible feats, but maybe it also means that he should have been by kageyama's side, because in those last three years kageyama started becoming aware of the distance between him and other players capability-wise and emotionally, and he lost the last person who wanted to keep up with him, his grandfather, and he was now truly alone.

king of the court and his right hand man

yuu-kun!